Insulation coating for indirectly heated cathode heaters



P 1965 J. c; HICKLE 3,206,329

INSULATION COATING FOR INDIRECTLY HEATED CATHODE HEATERS Filed Jan. 8, 1962 INVENTOR JAMES C. HICKLE ATTORNEY United States Patent 3,206,329 lNSULATiON COATKNG FOR INDIRECTLY HEATED CATHODE HEATERS James C. Hickle, Owensboro, Ky., assignor to General Electric Company, a corporation of New York Filed Jan. 8, 1962, Ser. No. 164,865 3 Claims. (Cl. 1172l5) My invention relates to heater elements for indirectly heated cathodes used in electron discharge devices and, more particularly, to insulation coatings for such heaters. More particularly, my invention relates to a cathode heater coated with a refractory material comprising an alumina silicate in the form of 3Al O :2SiO sometimes referred to as mullite.

Heretofore, heaters for indirectly heated cathodes have been coated with insulation in the form of a refractory material such as aluminum oxide or they have been coated with a mixture of refractory materials including aluminum oxide and talc. Such heaters are generally satisfactory but oftentimes the insulation becomes somewhat electrically conductive or leaky during the life of the tube and allows appreciable amounts of stray current to flow between the cathode and the heater wire. Such stray currents are undesired in that they introduce spurious electrical signals in the cathode circuit, which can produce unwanted hum or other deleterious effects.

Another disadvantage observed with the use of aluminum oxide coatings and the like for heaters is that the heater current tends to climb during the life of the tube, causing a change in the heating power and subsequent change in cathode temperature. Such rises in heater current are, of course, undesired not only because certain critical characteristics of the device are uncontrollably altered, but the useful life of the device is also shortened. I have found that if the heater is coated with mullite or mullite mixed with certain proportions of pure alumina, there is observed, first, a marked reduction in the magnitude of heater-to-cathode leakage when operating with the heater negative with respect to the cathode, and, second, a marked reduction in the rate of heater current rise when the heater is operated at a positive potential with respect to the cathode. Further, under the former condition, there is a marked reduction in the number of open-circuited heaters that are normally developed in a given period during the operation of the tube.

It is, therefore, a principal object of the invention to provide a new and improved heater construction for indirectly-heated cathodes.

Another object of my invention is to reduce the amount of unwanted and uncontrolled current leakage between the heater wire and cathode by coating the heater wire with a composition comprising mullite.

Another object is to reduce the heater current rise by using a mullite coating.

A further object is to reduce the probability of opencircuiting or burn-out of heaters during the operation of the tube incorporating the same.

In order that the invention may be clearly understood and readily carried into effect, reference may be had to the accompanying drawing and specific examples.

FIG. 1 is a perspective view of a part of a coated heater wire according to the invention.

FIG. 2 is a similar view of a heater wire having plural coatings, according to another embodiment of the invention, and

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2.

Referring to the drawings, which are not necessarily to scale, FIG. 1 illustrates a heater wire 1 which may be of tungsten or other suitable refractory metal, which is "ice coated with a mullite coating 2. The coating 2 may comprise mullite or it may comprise mullite mixed with pure alumina in various proportions as will be described in detail hereinbelow.

FIG. 2 illustrates the heater wire 1 coated, in accordance with another embodiment, with plural coatings. The layer 3 may be a coating of mullite and may, for example, be approximately 0.001 inch thick. The layer 4 may be an outer coating of pure alumina approximately 0.001 inch thick, the combined thicknesses of coatings 3 and 4 being between 0.002 to 0.003 inch. While coating thicknesses as here specified have been found to be satisfactory and successfully operable, it will be noted that coating thicknesses of either the mullite or the alumina, or both, can each be increased by a factor of 3 with equally beneficial results.

The coating of mullite is preferably disposed next adjacent the heater Wire with the alumina remote from the wire because the coefficient of thermal expansion of mullite is intermediate those of the wire and the alumina. Thus, on heating up and cooling down of the coated heater, as for example, during warm-up and cooling of the device, minimal thermal and mechanical stresses are experienced, reducing the likelihood of heater rupture during such cycling operations. The matching of thermal expansion characteristics of the wire and the coating can also be accomplished in the single-coating version of my invention, FIG. 1, as will be shown hereinafter.

FIG. 3 illustrates a cross-sectional view of a coated filament and brings out the relative positions of the layers 3 and 4 with respect to the tungsten heater wire 1 although, for convenience, the scale of the drawing has been distorted, showing the diameter of wire 1 to be many times that of the coating thicknesses that would normally be employed.

Commercially available pure aluminum oxide (99.6+ pure) can be used in preparing my insulating material for cathode heaters. The mullite used (3Al O :2SiO is an alumina silicate which is also commercially available, but which desirably is preliminarily leached with nitric acid or other suitable reagent to remove certain impurities such as magnesium oxide and iron oxide.

As has been previously stated, and in accordance with the embodiment of FIG. 1, my heater coating may contain 100% mullite or it may contain a mixture of mullite and pure alumina, the weight percent of mullite being in the range of from 10% to 100% of the mixture. The mullite may, in accordance with the embodiment of FIG. 2 be applied to the heater filament as .a first coating over which a second coating of pure alumina is applied.

While the theoretical reasons for the unexpected yet improved results derived from the mullite coatings are not fully understood, a probable explanation would involve the fact that mullite impedes the diffusion of oxygen ions to the wire through the coating. This impedance may be the result of the existence of stable sites for the oxygen atoms in the mullite crystalline lattice. It is known that oxygen ions cause the formation of oxides of tungsten which diffuse into the insulating coating causing the latter to become darkened. These ions :are released from the alkaline earth oxide coating of the cathode as a result of chemical and electrolytic reduction of the oxide. The oxygen ions, being negatively charged, migrate to the positive heater, diffuse through the coating and oxidize the tungsten. The tungsten oxide, being volatile at these temperatures, slowly migrates or difi'uses back through the aluminum oxide coating when such a coating is used. I have observed that when mullite is used in the insulating coating, the latter remains white, indicating that oxygen ions have not been .able to diffuse through the coating. As noted above, the avoidance of darkening of the heater insulation results in a reduction in heater current climb during the life of the device.

The observed low heater-to-cathode leakage is another beneficial result of the use of the mullite coating resulting from the fact that the crystalline structure of mullite provides stable sites for foreign atoms that normally would produce such leakage currents.

The reduction .in the number of open-circuited heaters is thought also to be related to the use of mullite in the heater coating. -In the first place, mullite has a lower thermal expansion than alumina and even when used with .alumina in the percentages indicated, operates to modify the expansion so that there is less of a mismatch of coefiicients of thermal expansion between that of the coating and that of the wire, usually tungsten. 'I have observed that in open-circuited heaters, the tungsten wire tends to fracture across grain boundaries under the insulating coating. The heater appears to be intact but the wire is usually parted and held in place by the insulation. It would thus appear that this effect is caused by migration of positively changed ions, which ions, as in the case of nickel, have a deleterious etfect on the properties of tungsten causing it to become brittle. When tungsten becomes brittle, the stress induced by the mismatch of the thermal expansion coetficients between the insulating coating and the tungsten result in fracture of the tungsten wire. These deleterious effects are avoided when, in accordance with the principal features of this invention, the tungsten wire is coated with mullite or with a coating composition containing mullite and aluminum oxide.

Specific examples of my coating composition are listed below, in which all percentages are by weight:

EXAMPLE I A mixture of finely pulverized mullite (30%) and alumina (70%) are added to Al(NO in a wa ter solution to form a slurry. The heater wires, formed of tungsten or alloys of tungsten and rhenium or thorium, are coated by dipping in the slurry and dried and then fired to sinter the coating in any conventional manner. Other conventional coating methods may be used, such as spraying and drag coating. Alternatively, the mullite and alumina may be ball milled in the presence of an organic solution containing a binder and the prepared solution may be sprayed or clectrophoretically deposited on the wire to form the insulating coating.

EXAMPLE 1 1 Using the above-described process, a slurry was made containing 10% mullite and 90% alumina. Tungsten wiresrwere coated as hereinbefore described.

Other coating compositions were made using substantially the same process and having the following pro- Separate slurries of pure alumina and mullite were prepared. The heater wire was preliminarily coated with mullite and then a coating of alumina was applied over the mullite coating. The mullite coating was approximately 0.001 inch thick and the alumina coating was also approximately 0.001 inch thick. The combined thicknesses of the mullite and the alumina coatings in various tests were between 0.002 and 0.003 inch.

In all of the examples described, the insulated heating elements were tested in electron discharge devices under 4!- various operating conditions up to 3000 hours of operation. It was observed, using pure alumina as a control, that the best results as to filament current climb and heater-.to-cathode leakage occurred when the mullite was present in the insulating coating compositions in amounts from 10-100%.

TABLE A FILAMENT CURRENT CLIMB ON LIFE [Tube type 6201 (E1=1=l2.6 v., Enx=+200 v. D.C.)]

Normal A120 hrs. 1000 2000 3000 Total coating (ma) hrs. hrs. hrs. change (ma) Una.) (ma.) (ma) 299 310 317 320 +21 311 305 308 313 +8 311 306 308 312 +6 20% mullite 311 305 318 +13 305 291 296 294 11 Mullite undereoat- 310 308 317 320 +12 Where EH is the applied heater voltage and E is the applied beaten to-cathode bias.

TABLE B [Tube Type 6222] 0 hrs. 500 hrs. 1000 hrs. Total Normal A1 03 coating (ma) (ma) (ma) change (Control) 169 181 190 +21 15% mullittL- 174 180 187 +13 30% mullitm 171 176 183 +12 Table C HEATER-CATHODE LEAKAGE ON LIFE [Tube Type 6201Heater Negative] Normal A1 0; coating 0 hrs. 1000 hrs. 2000 hrs. 3000 hrs.

(' 01a.) (h t (Control) 16 90 61 153 mullit 10 42 13 15% mullite 4 11 15 mu1lite 29 4 7 14 mullite 21 2 4 13 Mullite undercoat 5 3 1 13 Table A shows the reduction of heater current climb on life tests up to 3000 hours using varying proportions of mullite and the mullite undercoat for tube type 6201. It will be noted that substantial reductions in heater current climb are provided, notably in the case of 35 Weight percent mullite.

Table B indicates the results of similar tests on tube type 6222 up to 1000 hours. Here reductions of heater current rise of the order of 2:1 are available.

Table C gives the heater-to-cathode leakage current in microamperes on life tests from 0-3000 hours for tube type 6201, showing approximately a 10:1 reduction in such leakage current.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications can be made Without departing from the invention and I, therefore, aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In the fabrication of indirectly-heated cathodes for electron discharge devices to minimize undesired rise of heater current and heater-cathode electrical conductivity, and to avoid fracture of heater wire during normal operation of said devices, the method which comprises the steps of leaching a quantity of mullite powder to remove oxides of magnesium and iron that may be present as impurities, forming a slurry having not less than 10%, by weight, of said mullite and not more than by weight,

mullite of from 10% to 100%, by weight, and alumina of r from 90% to 0%, by weight.

3. The method as defined in claim 1, wherein said slurry comprises 100%, by Weight, mullite and further comprising an additional step of applying a layer of substantially pure alumina over the layer of mullite.

References Cited by the Examiner UNITED STATES PATENTS 1,826,510 10/31 Driggs 313 340 2,075,910 4/37 Robinson 313 340 2,112,969 4/38 Mavrongenis 313-340 X 2,964,668 12/60 Carter "313-345 X RICHARD D. NEVIUS, Primary Examiner.

10 J. W. HUCKERT, EARL Ni. BERGERT, Examiners. 

1. IN THE FABRICATION OF INDIRECTLY-HEATED CATHODES FOR ELECTRON DISCHARGE DEVICES TO MINIMIZE UNDESIRED RISE OF HEATER CURRENT AND HEATER-CATHODE ELECTRICAL CONDUCTIVITY, AND TO AVOID FRACTURE OF HEATER WIRE DURING NORMAL OPERATION OF SAID DEVICES, THE METHOD WHICH COMPRISES THE STEPS OF LEACHING A QUANTITY OF MULLITE POWDER TO REMOVE OXIDES OF MAGNESIUM AND IRON THAT MAY BE PRESENT AS IMPURITIES, FORMING A SLURRY HAVING NOT MORE THAN 90%, BY WEIGHT, ALUMINA IN A PRESELECTED VEHICLE, AND APPLYING THE LEACHED MULLITE AND ALUMINA TO A WIRE TO FORM A COATNG THEREON.
 3. THE METHOD AS DEFINED IN CLAIM 1, WHEREIN SAID SLURRY COMPRISES 100%, BY WEIGHT, MULLITE AND FURTHER COMPRISING AN ADDITIONAL STEP OF APPLYING A LAYER OF SUBSTANTIALLY PURE ALUMINA OVER THE LAYER OF MULLITE. 